This invention relates to a liquid crystal device, more particularly relates to a device which can be driven by application of electric voltage.
Smectic liquid crystal devices have been employed for displays of microcomputers, word processors, television systems and so forth, due to high contrast indicating ability. Also smectic liquid crystals have been considered promising as competitive media for memory devices such as disc memories and for applications to such audio-instruments as speakers.
There have been known a smectic liquid crystal device comprising a pair of substrates opposing to each other with a liquid crystal layer therebetween, a pair of electrodes provided on the opposed insides of the substrates and also a pair of oriented films symmetrically provided on the electrodes, in which a simple matrix structure or an active element structure with non-linear devices connected in series. A very important characteristic of such a liquid crystal device is a large coersive electric field Ec (threshold electric field). The large Ec makes it possible to hold the liquid crystal layer in an initial condition, for example, in an opaque condition, when the electric field applied to the layer is less than a certain strength. And, when the applied electric field increases to a level beyond the strength, the liquid crystal layer changes abruptly its condition into a transparent condition, and vice versa. In this regard, the coersive electric field Ec takes a positive value Ec+ (a theshold value observed when applied with a positive voltage), and a negative value Ec- (a threshold value observed when applied with a negative voltage). Although Ec+ and Ec- are not necessarily the same, both can be about equal in virtue of a processing condition of an orientation treatment on surfaces contiguous to a liquid crystal layer.
However, such a smectic liquid crystal layer exhibits a very faint hysterisis, namely, has very smalland unstable Ec+ and Ec-. Especially, in a smectic layer in a chiral C phase, the value of Ec depends largely on the strength of pulsed electric field applied to the layer. Hence, an excitation system known as AC bias method has been employed in which a negative pulse signal is applied in advance of rewriting in a positive direction, and then a positive pulse is applied under a finely controlling in terms of the strength of electric field applied and the applying time, and in inverse, when rewriting in a negative direction is desired, a procedure must be taken in a same manner in that the direction of electric field is reversed. The AC bias method makes a circumferential circuit very complicated.
Accordingly, it is required to make a liquid crystal device with a circumferential circuit being less complicated than that having an AC bias method. On the other hand, the bias method seems indespensible to implement a liquid crystal device as long as liquid crystal layer has only a slight Ec. There are same attempts to make a liquid crystal layer with a stable Ec. However, they are compromising its frequency characteristics.